1. Technical Field of the Invention
This invention relates to a focus detecting system, and more particularly to a focus detecting system which is employed for just focus detection in autofocus control of a taking lens. This invention also relates to a method for detecting a focus state of a taking lens. The foregoing methods may be implemented as a set of computer-readable instructions stored in a computer readable medium such as a data carrier.
2. Description of the Related Art
As a system for controlling focus by making a focus adjustment so as to maximize a focus evaluation value, what is called a hill-climbing mode has been proposed in which the focus is moved in a direction of increasing the focus evaluation value and stopped at a position where the focus evaluation value ceases to increase.
This hill-climbing mode is generally a system in which any one of front focus, back focus and just focus of the focus state is detected by wobbling of minutely moving the focus thereby to move the focus in the direction of just focus, i.e. of increasing the focus evaluation value. The hill-climbing mode, therefore, provides a fear that the change in the focus by the wobbling can be visualized on a screen. The hill-climbing mode presents a problem that the time taken until just focus is lengthened by the degree of wobbling.
On the other hand, for example, JP-A-2003-270517 discloses a focus detecting system which can detect the focus state without performing the wobbling using a plurality of imaging elements for focus detection having an optical path length difference.
In this focus detecting system, an object light incident on a taking lens is branched by a half mirror, and one of the branched lights is incident on an imaging plane of an imaging element for image production for acquiring an image signal for recording or reproducing (hereinafter simply referred to as an imaging element for image production), whereas the other (light for detecting the focus state) of the branched lights is further branched and incident on imaging planes of two imaging elements for focus state detection for acquiring the image signal for detection of the focus state (hereinafter referred to as imaging elements for focus state detection). These two imaging elements for focus state detection are located at the positions where on their imaging planes, the optical path is shorter and longer by equal distances than that on the imaging plane of the imaging element for image production. The focus state is acquired according to the magnitude relation of the two focus evaluation values which are calculated on the basis of the image signals acquired by the imaging elements for focus state detection. Specifically, the focus evaluation value acquired from each of the imaging elements for focus state detection corresponds to the focus evaluation value calculated on the basis of the image signal acquired by the imaging element for image production when the focus (focal point) of the taking lens is shifted from the present position to a near side and an infinite side by the equal distances. Thus, when these focus evaluation values are compared, if they agree with each other, it is determined that the focus state is just focus. If they are different, it is determined that focus state is front focus or back focus according to that either of these focus evaluation values is larger or smaller than the other. Thus, the focus evaluation values when the focus is shifted to the near side and the infinite side are simultaneously acquired by the image signals obtained from the imaging elements for focus state detection without actually moving the focus so that the focus state can be detected promptly without performing the wobbling.
Meanwhile, in the focus detecting system disclosed in JP-A-2003-270517, the imaging elements for focus state detection must be arranged so that their imaging planes provide a suitable optical path length difference. For example, if the optical path length difference is larger, the focus evaluation value obtained from each of the imaging elements for focus state detection represents the focus evaluation value obtained from the imaging element for image production when the focal point is largely shifted to the near side and the infinite side. If the optical path length difference is too large, the focus evaluation value at a low level is only obtained from either of the imaging elements for focus state detection in the vicinity of just focus so that in the case of out-of-focus, the difference between the focus evaluation values cannot be detected. On the other hand, if the optical path length difference is too small, the focus evaluation values obtained from the imaging elements for focus state detection are nearly equal so that the difference in the focus evaluation values cannot be detected. For this reason, where the focus of the taking lens is changed in the vicinity of just focus, it is desirable to set the optical path length difference so that the focus evaluation values obtained from the imaging elements for focus state detection largely vary in their opposite increasing/decreasing tendency.
However, in a case where the focal distance is variable like a zoom lens, when the focal distance is varied, the shifting quantity of the focal point corresponding to the optical path length difference in the imaging elements for focus state detection may vary largely. Namely, it has been confirmed that when the optical path length difference is fixed in a certain zoom lens, if the focal distance is set to be nearer to the side of a “wide lens terminal” (i.e. is made shorter), the shifting quantity of the focal point corresponding to the optical path length difference becomes larger. In such a case, a suitable optical path length difference cannot be set over the entire zooming range from the “wide lens terminal” to the “telescope terminal”. Even if the optical path length difference is suitable on the side of the telescope terminal, it may be too large on the side of the wide lens terminal. Inversely, even if the optical path length difference is suitable on the side of the wide terminal, it may be too small on the side of the telescope terminal. This leads to a case where the difference in the focus evaluation values cannot be detected. This invention has been accomplished in view of such a circumstance.